Passive Radio Frequency ID (RFID) devices are broadly defined as radio frequency transponders that are activated and powered by the RF field of a remote detector or reader. Common examples are electronic anti-shoplifting (EAS) tags sold by Checkpoint Systems of Thorofare, N.J. An EAS tag is affixed to a retail item and the movement of the item into the field of the RF detector may be sensed by the absorption of RF energy by the resonant circuit in the tag. In more advanced systems a small integrated circuit (IC) may be incorporated into the circuit. This IC is powered by the remote RF field and may respond to the field by broadcasting data that may be interpreted by the reader. The data may be a unique serial number to identify the item or a more complex data set. There are a number of such commercial systems available for tracking and inventory applications.
FIG. 1A-1B show the conductive layers for a prior art radio frequency sensor. FIG. 1A shows a top conductive layer. This top conductive layer is positioned over a substrate. FIG. 1B shows the bottom conductive layer. The top layer shows the conductive coil 102 and the top plates 104A and 106A of capacitor regions. These capacitor regions also include the bottom plates 104B and 106B shown in FIG. 1B and the intervening substrate.